1. Field of the Invention
The present invention relates to a color image processor for converting a RGB data composed of respective Red, Green and Blue color separated signals used in a color input device into a CMYK data composed of respective Cyan, Magenta, Yellow and Black signals used in a color image recording system and, particularly, to an optimization of an output of a color document containing character images and picture images in a mixed state.
2. Description of Related Art
A color input device such as scanner, monitor or digital camera, etc., usually uses the RGB color space for a color management. On the other hand, a color management of a color output device such as printer or copier, etc., is performed by using the CMYK color space. Therefore, a color space conversion is required between the color input device and the color output device.
In general, the conversion of the RGB data into the CMYK data is achieved by, first, converting R, G and B colors into C, M and Y colors with using the complementary color conversion or the Look-Up-Table (LUT) for color conversion and, then, obtaining K (black color) information by performing the Black-Generation and Under-Color-Removal (BG/UCR) processing. In this specification, the data generated from R, G and B color information with using the LUT is referred to as C′M′Y′ data and the data generated from the C′M′Y′ data with using the BG/UCR processing is referred to as CMYK data.
The BG/UCR processing is advantageous in view of improvement of an image contrast and restriction of toner consumption, etc. However, it is detrimental in view of degradation of chromaticness in high chromaticness region. Further, since toners for C, M and Y colors have no ideal chromogenic characteristics, a balance in concentration between the CMY data and K data is degraded with increase of the amount of the BG/UCR processing.
It is preferable, in order to maintain the sharpness thereof, to print a character image by using only the K (black) plate and, therefore, the BG/UCR processing is indispensable for the character image printing. On the other hand, in a case of an image such as a picture image, which requires the continuity of gradation of chromaticness as well as colors, it is preferable to perform a color reproduction by C, M and Y colors for substantially entire input gradation values. In such case, the BG/UCR processing may generally restrict the picture image.
In outputting a color document containing a character image and a picture image in a mixed state, it is desired that the character image is reproduced by monotone using the K plate and a substantial portion of the picture image is reproduced by C, M and Y colors simultaneously with the black character image. However, when characters are to be printed by using only the K plate, it becomes necessary to perform the BG/UCR processing for the black solid input C′M′Y′data such that the data is replaced by data of the K plate. Since the BG/UCR processing is also applied to the picture image, a black solid portion of the picture image is reproduced by monochromatic black color of the K plate. That is, since the black solid portion in the picture image is reproduced by the K color while the other portion of the image area than the black solid portion is reproduced by C, M and Y colors or C, M, Y and K colors, there is a clear gap in hue, chromaticness or lightness in a boarder area between the black solid portion and the other portion depending upon the chromogenic characteristics of the C, M and Y toners. In order to solve this problem, it is necessary to restrict the amount of the BG/UCR processing for the black solid input C′M′Y′ data to thereby remove portion or portions of the image, which are reproduced by using only the K plate. In such case, however, it is, of course, impossible to reproduce the character image by only K color.
There have been many proposals for performing the BG/UCR processing appropriate for low chromaticness region and the BG/UCR processing appropriate for high chromaticness region. For example, Japanese Patent Application Laid-open No. H7-087346 discloses a technique in which an information of chromaticity, lightness and chromaticness is obtained by converting an input RGB data into a L*a*b* data, an UCR rate is determined by non-linearly operating a chromaticness signal and four color image output signal including K form is determined on the basis of the UCR rate and the L*a*b* value. Further, Japanese Patent Publication No. H7-36609 (corresponding to Japanese Patent Application Laid-open No. H4-317259) discloses a technique in which a chromaticness information is obtained by an input value and, after the BG/UCR processing is performed correspondingly to the thus obtained chromaticness information value, amounts of C, M, Y and K are corrected again by performing the Under-Color-Addition (UCA) processing. With using these techniques, it is possible to simultaneously realize the reproduction of a character image with only K color and the restriction of amount of the BG/UCR processing for a high chromaticness region of a picture image.
However, since, in these prior arts, the BG/UCR processing optimized for low chromaticness region is performed in a solid black portion or a portion close to solid black portion of a picture image, the solid black portion, etc., is reproduced with only K color and a peripheral portion thereof is reproduced with C, M, Y and K colors. Therefore, the problem of the gap in hue, chromaticness or lightness can not be solved. Further, since any of the mentioned prior arts has developed under consideration that simultaneous realization of high quality reproduction of both colorless region and colored region is the highest priority, problems of increase of a data processing time and increase of F/W memory capacity are left as they are.
Further, in outputting a color document containing a character image and a picture image in a mixed state, it is possible to reproduce the characters with using only the K plate by optimizing the BG/UCR processing therefor. However, such optimized BG/UCR processing is not always appropriate in reproducing the picture image. That is, since the solid black portion of the picture image is reproduced by only the monotone of the K plate, there is a gap in color, chromaticness or lightness in the boarder region between the solid black portion and other portion printed with three or four colors, which are contained in the picture image. This is because the chromogenic characteristics of toners of C, M and Y colors are not ideal and the balance in concentration between C, M and Y colors and the K color is broken when the amount of the BG/UCR processing is increased, as mentioned previously. Therefore, in order to avoid such gap, it has been necessary to sacrifice the quality of either character or picture.